Prilling and spray drying are used widely in the production of spherical particles having uniform size and physical characteristics. Prilling is particularly useful in the fertilizer industry. The advantages of prilling are well known in the art, and include: high percentage of desired product size and thus little recycle, reduced moisture content leading to reduced drying, and excellent sphericity. One frequent problem in prilling relates to processing thick mixtures resulting from a high concentration of a non-melting component in a melt. It may become difficult to flow the resulting thick mixture through conventional prilling systems.
U.S. Pat. No. 7,175,684 addresses this problem by effecting shear thinning by mechanical agitation in the prill head. The thinned mixture can be prilled through a modified prill head in a conventional manner. The features and disclosures of the '684 patent are incorporated herein in their entirety by reference thereto.
Other approaches have been disclosed to solve this problem of a thick mixture. For example, methods have been devised to manage this complication by minimizing reaction time. GB 1,481,038 teaches a simple concept of severely limiting the processing time (the period between the time the melt is fed into the mixer to the time the drops are discharged from the prilling apparatus) to 10 seconds or less so that the mixture can be prilled before any detrimental effects develop. U.S. Pat. No. 3,617,235 teaches the use of larger-sized solid particles to slow reaction prior to prilling. U.S. Pat. No. 4,323,386 teaches a method of managing the addition of reagent, delaying full addition of ingredients until just prior to prilling, again to preclude reactions. U.S. Pat. No. 3,856,269 discloses a mixing apparatus to facilitate prilling by providing very rapid but adequate mixing of fertilizer ingredients prior to prilling in a standard perforated spinning bucket. These methods add cost and complication to the prilling operation, however, by requiring feedstocks with narrow specifications or the expense of engineering the production system to achieve very brief residence times, or else compromising a desirable property by curtailing the extent of reaction.
Another approach to prilling thick mixtures is to provide equipment that will force the flow of the slurry. SE 70,119 teaches a vertical screw machine to blend ammonium nitrate melt with ammonium sulfate solid; the pressure developed by the screw and static head is supplemented by injection behind the spray nozzle. DE 2,355,660 teaches a prill head incorporating a stirrer-impeller mechanical device similar to a centrifugal pump whereby the slurry is introduced at the center and forced under pressure out holes on the circumference of the prilling disk. These mechanical devices involve considerable cost in construction, and the abrasive nature of fertilizer slurry will wear the close clearances necessary for efficient pumping with resulting significant maintenance expense.
Spray drying using a two-fluid nozzle is disclosed in U.S. Pat. No. 4,705,535. The nozzle is adjustable to provide a substantially constant mixing energy for atomization of the liquid. Mixing energy is determined from the mass flow of gas and of liquid to the nozzle, gas pressure at the point of feed to the nozzle, and pressure in the vessel into which the spray is discharged. This reference is silent, however, with respect to modification of characteristics of the fluid to the nozzle.
U.S. Pat. No. 4,013,504 teaches a spray-drying apparatus comprising a high-speed rotary atomizing head having circumferentially spaced discharge orifices and spiral vanes on the exterior portion of the head. A film of slurry from the orifices flowing over the vanes is subjected to high shear stresses which materially improve the viscosity of the product and disperses particles in an efficient manner. This reference acknowledges the problem of viscosity and use of shear, but does not disclose control of flow through the orifices.
The present invention avoids the higher costs, maintenance and complications of the known art as well as the disadvantages of approaches based on curtailed reactions, and builds on the shear-thinning feature of U.S. Pat. No. 7,175,684.